Because of the small sizing of the casing or bore diameter of injection and production oil recovery wells, down hole heaters, if used, have heretofore relied on electrical heating elements inserted into the casing. Whether the heating elements be resistance heating elements or induction heating elements, the power generating equipment must be capable of generating high heat fluxes. The space limitations within the casing make it difficult to position and size electrical heating elements which can generate high heat flux uniformly along the casing lengths. In fact, the heating elements gradually heat the steam or water travelling along the length of the elements to higher and higher temperatures until steam is formed at the discharge point. Thus, down hole heaters use excessive amounts of electricity to generate high heating fluxes in applications where heating progresses to the highest temperature coincident with the discharge point of the steam from the heater. Those types of heaters are clearly inapplicable to the oil recovery system claimed in the U.S. Pat. No. 5,020,596 grandparent patent or to any other recovery scheme where a uniform heat flux is required over a length portion of the heater.
Fuel fired burners are, from an energy cost analysis, less expensive than electrical heating arrangements. However, the size of the well casing coupled with the requirement that hot water or steam be generated or boosted at the bottom of the casing while the steam or water flows therethrough has heretofore precluded their application as heaters for recovering materials from subterranean formations.
Radiant tube burners or heaters have long been used in industrial heating applications and have conventionally been powered by electrical heating elements or by fuel fired burners. Electrically heated radiant tubes basically comprise heating elements within a tube which extend into a furnace or work zone. The elements radiate heat to the tube and the tube radiates heat to the work. In high temperature heating applications such as those involving the melting of metals and the like, electrically heated radiant tubes are preferred since the heating elements radiate uniform heat flux to the tube. Again, the cost of electricity in a high temperature flux application dictates that fuel fired burners be used to fire their products of combustion into a tube which in turn will radiate heat to the work. However, fuel fired radiant tube heating applications do not maintain a uniform temperature along the length of the tube especially at high temperatures where radiated heat fluxes are especially significant when considering heat transfers from burner to work. In such application, the adiabatic temperatures produced by the fuel fired burner cause a hot spot whereat the heat flux intensity is greater than that at other areas of the burner. Numerous schemes have been tried to arrive at uniform distribution heat patterns, especially at high temperatures from fuel fired burners. These have met with varying degrees of success. One such arrangement, funded by Gas Research Institute, uses a tangentially fired burner with products of combustion from the burner entering a slotted baffle arrangement to develop high convective heat transfers in the form of slotted jets. Convective heat transfer from the slotted jet is then used as a "boost" to the radiated heat flux from the tangential burners to heat a mantle to very high temperatures of 2500.degree. F. However, the heat transfer coefficient while enhanced with this arrangement is fundamentally limited by the coefficient attributed to the radiation heat transfer of the tangentially fired burner which is poor.
Also, within the industrial burner art there are numerous fuel fired burner arrangements which, at first glance, might bear some structural resemblance to the fuel fired radiant tube heater of the present invention, but which have entirely different functions and purposes associated with the structure. For example, Bark U.S. Pat. No. 3,946,719 discloses a burner with longitudinally spaced apertures designed to receive combustion air for cooling certain burner parts to prevent thermal breakdown of the burner.